Liquefaction of gas, particularly natural gas, is well known from larger industrial plants, so called “baseload” plants, and from peak shaving plants. Such plants have the property in common that they convert a substantial quantum gas pr unit of time, so they can bear a significant upfront investment. The costs pr, gas volume will still be relatively low over time. Multi-component refrigerants are commonly used for such plants, as this is the most effective way to reach the sufficiently low temperatures.
Kleemenko (10th International Congress of Refrigeration, 1959) describes a process for multi-component cooling and liquefaction of natural gas, based on use of multi-flow heat exchangers.
U.S. Pat. No. 3,593,535 describes a plant for the same purpose, based on three-flow spiral heat exchangers with a an upward flow direction for the condensing fluid and a downward flow direction for the vaporizing fluid.
A similar plant is known from U.S. Pat. No. 3,364,685, in which however the heat exchangers are two-flow heat exchangers over two steps of pressure and with flow directions as mentioned above.
U.S. Pat. No. 2,041,745 describes a plant for liquefaction of natural gas partly based on two-flow heat exchangers, where the most volatile component of the refrigerant is condensed out in an open process. In such an open process it is required that the gas composition is adapted to the purpose. Closed processes are generally more versatile. There is however, a need for liquefaction of gas, particularly natural gas, many places where it is not possible to enjoy large scale benefits, for instance in connection with local distribution of natural gas, where the plant is to be arranged at a gas pipe, while the liquefied gas is transported by trucks, small ships or the like. For such situations there is a need for smaller and less expensive plants.
Small plants will also be convenient in connection with small gas fields, for example of so called associated gas, or in connection with larger plants where it is desired to avoid flaring of the gas. In the following the term “product gas” is used synonymously with natural gas or another gas to be liquefied.
For such plants it is more important with low investment costs than optimal energy optimization. Furthermore a small plant may be factory assembled and transported to the site of use in one or several standard containers.
U.S. Pat. No. 6,751,984, by the same applicant as the present invention, describes a concept for small scale liquefaction of product gas. The concept is based on two-flow heat exchangers with a downward flow direction for the condensing fluid and an upward flow direction for the vaporizing fluid. The cooling is taking place at essentially one pressure level. The shortcoming of this process is however that it requires many heat exchangers for realizing the process, and at least two primary heat exchangers serially connected for condensing the product gas. This makes the process somewhat complex and then less suitable for use in some applications.